The present invention relates to the protection of the surface of iron and steel products shaped at a relative)y high temperature.
A preferred application of the invention concerns long products and, in typical, but non-limiting manner, the protection of reinforcing bars, hereinafter referred to as rebars.
The temperature at the end of working off of the rebars leaving the finishing stands of the rolling mills is usually of the order of 900.degree. to 1000.degree. C. The rebars are then cooled, placed in readiness, then taken up to be cut to length.
In current practice, the rebars are not considered as noble products and do not undergo any protection treatment.
However, an increasing need for rebars protected against corrosion is observed at present. For certain markets where storage in the open air lasts a long time (e.g. six months) and takes place in a relatively corrosive atmosphere such as that of a port, an at least temporary protection of the rebars is desirable. In other cases, a better resistance to corrosion in service is demanded of the rebars used in works made of a particular concrete, of which certain constituents are more particularly liable to corrosion, or made in a relatively aggressive environment. In those cases, a protection of long duration is desirable.
To respond to this increasing need, it is necessary to find an efficient and economically acceptable process of protection. This latter criterion is all the more strict as the market value of rebars per se is very low.
The solution, which has been proposed, of making the rebars from stainless steel, entirely or plated therewith, can thus not be generalized due to its cost.
The processes known to the man skilled in the art for protecting the iron have, up to the present time, not found a commercially advantageous application for rebars, as they all require a step for preliminary preparation of the product in order to remove the scale and other oxides developed on the surface during cooling of the product, during manufacture thereof.
In this way, hot galvanizing requires, apart from exceptions (JP-A-54/133438), a prior pickling of the rebar with acid, as the scale forms a barrier to the reaction of galvanizing. The scale also opposes an electrolytic deposit or the application of a film-forming material, preceded by the formation of a layer of finish, either by heat treatment (U.S. Pat. No. 3,085,034) or by reaction of the bare surface with an acid solution of metal sulfates (GB-A-1 153 202) or with vapor (Review of Current Literature on the Paint and Allied Industries, Vol. 22, No. 129, May-June 1949, page 265).
Similarly, metallization by spraying (called "schoopage"), in particular metallization with zinc, is known to the man skilled in the art to require pickling, by sand-blasting or shot-blasting, of the surface to be treated (cf. "Techniques de l'Ingenieur", M1641-4, para. 3,6; "La Metallisation du zinc", page 5, para. 5 and 13, para. 9, published by the "Centre Technique
du Zinc"). It is therefore systematically sought to obtain on the unworked product scale which adheres the least possible in order to facilitate removal thereof. Now, such pickling requires that the rebars produced be taken up individually or in very small groups, this therefore considerably increasing the cost of the treated rebars.
Similar drawbacks encumber the process disclosed by FR-A-2 029 285 according to which there is applied to the surface to be protected, which is still very hot, a metal borate which brings about a reaction of dissolution of the scale, which produces a brittle layer which must be subsequently detached by a mechanical means.